Well if you're building it from scratch, then all you would need to do is attach the axle of the wheel to a fixed spring---I think you pretty much have it. If you don't insist on building it from a 2x4, then there are actually a bunch of exercise machines for leg presses which basically operate on the same principle, take for example the bowflex (where the bows basically function as the spring): http://www.socialshopping.com/image...s--Bowflex-PR1000-Gym-Fitness-equipments-.jpg The seat can roll along the rail it's on, but the bows pull the seat back in.

Thanks Jolb - i'm thinking on a much smaller scale though. The ball or wheel is only about an inch or two in diameter. And no, i don't want do it from scratch. I would much rather go to Lowes or order it online if possible.

I just need a rolling ball or wheel that will compress. These MIGHT work, but i've never used one before: http://www.te-co.com/Ball+Plungers-C-3001-C-.aspx [Broken]

What about a metal tape measure. you pull it out and release it snaps back in. There is also small ones for your keys so you don't loose them. Or a extendable leash for a dog. Just tie the loose end to the roller and the other end somewhere on your 2x4.

Staff: Mentor

How many wheel rotations do you want to have between the spring being relaxed and at full compression?

There are some toy cars that have a spring mechanism attached to their rear wheels so that you can rub them backwards on the ground a bit, and then release them to let the spring energy propel them along for a while...

No, i'm sorry. I'm not wanting any kind of spring for the wheel to wind. The wheel should spend freely. I just want the wheel to be able to be pushed down. Just like this product: http://www.allaboutdoors.com/product...ducts_id=25350 [Broken] - only if something pushing down on the wheel that it will sink in and then spring back up.

Staff: Mentor

No, i'm sorry. I'm not wanting any kind of spring for the wheel to wind. The wheel should spend freely. I just want the wheel to be able to be pushed down. Just like this product: http://www.allaboutdoors.com/product...ducts_id=25350 [Broken] - only if something pushing down on the wheel that it will sink in and then spring back up.

Can you just add your own springs to that wheel assembly? Although it might be good to use one with 4 mounting holes instead of 2, to add stability to the suspended assembly.